Identification of candidate genes for rice root growth under drought conditions

The identification of genes Involved in variation of rice root growth would assist breeders in creating new cultivars with improved drought avoidance. Out of 121 unique transcript-derived fragments (TDFs), which differentially expressed in three root sectors (primary root tips, lateral roots and adventitious root primordial zones) from an upland tropical variety Azucena under drought condition, 113 were anchored to the rice high-density linkage maps using their nucleotide sequences. Eighteen were in silico mapped on the interval with the common quantitative trait loci (QTLs) for root traits in rice. Furthermore, nine of these TDFs were mapped onto the rice molecular linkage map using a recombinant inbred line population derived from a cross between IR1552 and Azucena. All of them were located in regions where QTLs associated with root growth under drought are known to lie. Northern blot analysis showed that nine genes exhibited different expression patterns in three root sectors between parent cultivars tested with respect to drought regulation. These nine were the putative candidate genes for root growth under drought condition, while they were highly similar to cDNAs encoding known proteins including two expansins (OsEXP1 and OsEXP2), two xyloglucan endotransferases (XET), an endo-glucanase (EGase), a SART1 protein, a cytochrome P450 monooxygenase, a stomatin-like protein and an aci-reductone dioxygenase. Among the nine candidates, five genes (OsEXP1, OsEXP2, two XET and EGase) are known to be involved in expansive growth of cell walls, indicating that drought could stimulate root cell growth.